There is known a motor vehicle steering system including a transmission ratio variable mechanism capable of changing a transmission ratio of an output rotation angle to an input rotation angle (for example, see Patent Documents 1 and 2).
In the motor vehicle steering system in Patent Document 1, between an upper steering shaft coupled to a steering wheel and a lower steering shaft coupled to a steering gear, there is arranged a toothed-gear reducer including toothed gears of which tooth sections are formed of “rollers.” In each “roller” the both ends are held by a retainer, and are rotatable to a toothed gear main body.
The transmission ratio variable mechanism arranged in this toothed-gear reducer is provided with a first toothed gear of which the rotation is restrained by the upper steering shaft, a fourth toothed gear of which the rotation is restrained by the lower steering shaft, and a swinging toothed gear having an axis inclined to rotation axis of the first and fourth toothed gears. At the both ends of an outer toothed gear section of a swinging toothed gear, there are formed second and third toothed gears, and the second toothed gear is meshed with the first toothed gear and the third toothed gear is meshed with the fourth toothed gear.    Patent Document 1: Japanese Published Unexamined Patent Application No. 2006-82718    Patent Document 2: Japanese Published Unexamined Patent Application No. 2007-170624